The present invention generally relates to casting processes and the equipment and materials used in casting processes. More particularly, the invention relates to a metal casting process adapted to simultaneously form an oxidation-resistant surface region in a cast product produced by the process, which can otherwise be a conventional casting process such as, but not limited to, iron-based alloy casting and sand casting processes.
Internal components of gas turbines, including gas turbine engines used to the power generation and aircraft industries, must be capable of operating at high temperatures, often above 1000° F. (about 540° C.). Because the efficiency of a gas turbine is dependent on its operating temperatures, there is a continuing demand for gas turbine components with higher temperature capabilities. As the material requirements for gas turbine components have increased, various processing methods, alloying constituents and coatings have been developed to enhance the mechanical, physical and environmental properties of gas turbine components.
To promote oxidation resistance and acceptable operating life, internal components of gas turbine engines are often produced as castings from high-alloy stainless steels, nickel and cobalt alloy steels, cobalt-based alloys and nickel-based alloys. These alloys rely on chromium and/or nickel as alloy additions to form an oxidation-resistant scale as a surface layer capable of protecting the alloy from high temperature and oxidizing conditions within gas turbines. The scale is predominantly chromium oxide (chromia; Cr2O3) and nickel oxide (NiO) as a result of the chromium and nickel contents of these steels, for example, about 18 weight percent or more of chromium and about 8 weight percent or more of nickel in austenitic stainless steels, about 20 weight percent or more of chromium in nickel and cobalt alloy steels, nickel-based alloys and cobalt-based alloys, and about 50 weight percent or more of nickel in nickel-based alloys. The scale effectively passivates the component surface, preventing further corrosion at the component surface and inhibiting corrosion beneath the surface. Though providing the advantage of oxidation resistance, these alloys are typically much more expensive than low-alloy steels and irons, which typically contain less than 10 weight percent chromium and little if any nickel.
It is well known that an aluminum oxide (alumina; Al2O3) scale can offer superior protection over chromium oxide and nickel oxide in many high temperature applications. Certain nickel-based superalloys, for example, René N5 (U.S. Pat. No. 6,074,602), contain a sufficient amount of aluminum to promote the growth of a stable alumina scale. However, the addition of aluminum to molten steel alloys during the initial manufacturing process is difficult and often impractical or uneconomical.
As an alternative, aluminum can be diffused into the surface of a component using a diffusion process, such as pack cementation, vapor phase (gas phase) aluminiding (VPA), or chemical vapor deposition (CVD). Such diffusion processes generally entail reacting the surface of the component with an aluminum-containing vapor to form an additive layer at the component surface and a diffusion zone beneath the additive layer. The additive layer typically contains an environmentally-resistant intermetallic phase MAl (where M is iron, nickel or cobalt, depending on the substrate material), and the diffusion zone typically contains various intermetallic and metastable phases of aluminum and elements present in the substrate. While effective, diffusion aluminide processes require an additional processing step after casting that includes a thermal treatment, which for some applications is impractical and in all cases incurs additional cost and time.
In view of the above, there is an ongoing desire to produce iron-based alloy cast products that contain relatively low levels of alloying constituents, yet are capable of exhibiting adequate oxidation resistance for use in high temperature oxidizing environments, as is the case with internal components of gas turbine engines.